Drilling muds useful in drilling trona beds



2,957,822 Patented Oct. 25, 1960 DRILLING MUDS USEFUL IN DRILLING,

TRONA i William R. Frint, Green River, Wyo., assignor to Food Machineryand Chemical Corporation, New York, 'N.Y., a corporation of Delaware NoDrawing. Filed Nov. 27, 1957, Ser. No. 699,175

3 Claims. (Cl. 2528.5)

This invention pertains to a process of drilling through beds of trona(natural sodium se'squicarbonate) such as encountered in the Green Riversection of Wyoming,.the salt-based drilling mud composition utilized, inthis process,

and the aqueous salt composition used to formulate the drilling mud usedin this process.

It is well known that in the rotary drilling of boreholes an aqueoussuspension of finely divided solids, commonly referred to as drillingmud, is pumped down the drill stem through the openings in the drill bitand upwardly through the annular space between the drill stem and the vwalls of the borehole to the surface of the earth. A primary purpose foremploying the drilling mud is to pick up the cuttings produced by thedrill bit and to transport.

these cuttings to the surface of the earth. The drilling mud also servesother important functions such as lubricating the bit and the drillstem, cooling theibi t, andfur nishing a hydrostatic pressure head toprevent flow. into the borehole of formation fluid,.such as oil, gas andwater,

preferable that the filter cake. formed on the walls of the.

borehole be thin rather than thick so asto ,avoid mechanicaldifliculties in moving the bit inand out of thenhole and in placingcasings in the hole.

While the science of drilling mud formulation is well developed for thedrilling of the borehole through common rock and shale formations,difiicultie'sare encountered when attempting to drill throughwater-soluble salt formations due to the tendency of the salt todissolve into aqueous drilling mud. This causes uneven boreholes andconsiderable caving of the boreholewalls. i j

It is customary when drilling through water-soluble salt formations toutilize a salt based. mud formulated with a saturated aqueous solutionof the saltcompoundedrwith special clays which retain colloidalproperties in the high salt concentrations and mixtures of organiccolloids such as starch and carboxymethylcellulose. Such salt-baseddrilling muds are disclosed in US. Patent Nos. 2,525,783, 2,571,093 and2,771,419.

In the process of drilling wells for the solution mining of trona beds,which in the Green River area of Wyoming are situated at various layersup to 12 ft. in thickness under 800 to 1500 feet of overburden, it iscustomary to drill through the cap-rock and trona bed to the underlyingstrata. The borehole is then cased with pipe and the casing cemented tothe formation. If considerable solution of the trona bed has occurredaround the borehole, the amount of cement required to seal the hole isexcessive. After the hole is sealed, the cement plug is reamed through.Hydraulic pressure is then applied to the area through the casing tocause hydrofracturing of the strata at the interface of the underlyingstrata and the trona bed. The pressure forces the trona bed to raiseslightly for a fires Patent considerable area around the hole. Adjacentwells are thus connected so that water or other solvent can flow frominput to output wells, which water or solvent dissolves the trona as itflows underground. If the casing is not firmly cemented to the-tronastrata before applying hydraulic pressure, the water would flow back upbetween the casing and the trona bed along the uneven boreholes and whenpressure was applied it would either force the water out of the boreholethrough the annulus outside the casing or if pressure could bemaintained, this pressure would not be applied at the strata interfacebut through out the entire bed, causing vertical rather than horizontalfracturing with no interconnection of the wells.

I have found that the use of a saturated trona brine formulated with asalt resistant clay of the nature of Florida-Georgia zeolitic clays(floridin), starch and the sodium salt of carboxymethylcellulose, aswould be suggested by the prior art, is unsatisfactory for drillingthrough trona beds. The amount of trona that water can dissolve variesconsiderably with the temperature of the water. A drilling mudsuspension which is saturated with respect to trona at the mud pitundergoes considerable heating during the process of pumping it throughthe drill stem through heat exchange with ascending mud which has beenwarmed by removing heat generated by the drill bit. Thus, the solutionwhich is saturated at the surface temperature is not saturated withrespect to trona when leaving the drill bit and will dissolve trona fromthe bed to reach the saturated point I have further found that the useof a saturated salt 11.; (sodiumchloride) drilling mud formulated asabove is also disadvantageous in that solution of the trona from. thetrona beds likewise occurs. The saturated salt solution is notsaturated'with'respect to trona and the amount of trona which such asaturated salt drilling mud will disjsolve will likewise increase withan increase in temperature of the mud.

While the use of a trona brine mud is much more advantageous than theuse of an aqueous mud or a saturated salt mud, it still suffers thedrawback of uneven boreholes and caving of the formation. Excessivecementing is rewhich will enable the drilling of boreholes through bedscontaining trona as exemplified by the trona beds in the Green Riversectiouof Wyoming without solution of the embedded trona into thedrilling mud.

It is a further object of our invention to obtain a borehole through atrona bed which is even in diameter and into which a casing can becemented without use of more than minimum amounts ofcement. i

It is still a further object of our invention to obtain cores bycore-drilling in trona formations, which cores show no dissolution ofthe trona contained therein.

I have discovered that a salt-based drilling mud in which the major saltconstituent is sodium chloride with minor amounts of sodium carbonateand sodium bicarbonate, does not dissolve trona present in the formationover a wide range of temperature, thus enabling one to drill throughnatural trona beds without solution of the bed and allowing one toobtain a borehole having practically uniform diameter as shown by acaliper log of the borehole.

Any of the customary salt-based drilling mud formulations can be used inthe operation of our invention. In place of concentrated sodium chloridebrine usually used as a liquid base in these formulations, I substitutea brine comprising 20 to 24% of sodium chloride, 3 to 7% of sodiumcarbonate and 0.6 to 3% of sodium bicarbonate. In the presence of abrine of the above composition, natural trona is insoluble over a widerange of temperatures from C. to 90 C. and only slightly soluble attemperatures above this. The above composition is compatible with theingredients customarily utilized in salt-based drilling muds.

To our chloride-carbonate-bicarbonate brine can be added various clayswhich retain their colloidal properties in the presence of sodium ions.Such clays are wellknown in the art, a commercial clay of this type isZeogel sold by the National Lead Company. Natural clays such as theFlorida-Georgia zeolitic clays known as floridin can also be used. Ifthe clay does not add sufficient colloidal properties to cut down onwater-loss to the formation, organic colloids can be added in addition.Such organic colloids are, for example: water-soluble starch and thesodium salt of carboxymethylcellulose. Weighting agents can also beadded, although these are not necessary in the drilling muds utilized inthe Green River area of Wyoming. When using starch, bactericides such asformaldehyde may also be used.

I have found that an excellent mud can be formulated from Zeogel,water-soluble starch, sodium carboxymethylcellulose, and the dissolvedsalt-solution, which has a density of 10.7 lbs./gal., a water loss of 3cc. in 30 minutes (API), a pH of 1 0.5 and a viscosity of 5 8 seconds(API). Use of this mud enables one to drill through trona formations andproduce a hole diameter of approximately the same size as the bitdiameter. The cores obtained utilizing this drilling mud show noevidence of dissolution of the trona.

Example A drilling mud was prepared having the following com position:

As the mud is fairly expensive, a plain water mud was used to drill downto the sandstone (800 ft.). Then the special mud was used for theremainder of the drilling, core drilling and reaming. The cores obtainedfrom the various trona formations showed no dissolving of tronawhatever, whereas all previous cores had partial or complete dissolutionof the trona, depending upon whether trona liquors or water had beenused in the drilling mud.

In addition to furnishing an ideal drill core, the finished hole wasexceptionally good. Practically no sluifing of the various formationsinto the drill hole had taken place. Caliper logs of the wells drilledwith this drilling mud composition were made, which show the actual holediameter versus depth. In general these logs show that during the coringthe hole diameter is exactly the same as the bit diameter throughout thecored section. In drilling with ordinary mud above the 800 ft. level,the hole diameter is often more than 2 inches and generally at least 1inch larger than the bit diameter. When the special mud was used, seldomwas the hole diameter more than 1 inch larger than the bit diameter andmost of the time they coincided. It is desirable to have a uniform drillhole for two reasons. Less cement is needed when cementing the casingand the cement will be much stronger. If there are large cavities wheresluffing of the formation has occured the mud will not be replaced bycement during the cementing and the casing will have a weakened cementjob as a result.

While we have given specific embodiments of our invention, we Wish it tobe understood that the invention is not limited to these embodiments andthat various changes and modifications can be made therein withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

I claim:

1. A salt-based drilling mud for use as a circulating fluid in a well indrilling through rock formations containing trona, said mud containingcolloids selected from the group consisting of clay and organiccolloids, and a salt-based aqueous brine comprising about 20% to 24% ofsodium chloride, about 3% to 7% of sodium carbonate, about 0.6% to 3% ofsodium bicarbonate, and the remainder water.

2. A salt-based aqueous brine for use as a base in the formulation ofdrilling muds useful as circulating fluids in a well in drilling throughformations containing trona deposits comprising about 20% to about 24%of sodium chloride, about 3% to about 7% of sodium carbonate, about 0.6%to 3% of sodium bicarbonate, and the remainder water.

3. In a process for drilling a well through formations containing tronadeposits with well drilling tools wherein there is circulated in a wella salt-based drilling mud containing colloids selected form the groupconsisting of clay and organic colloids, the improvement which consistsin circulating in said well a salt-based drilling mud compounded with asalt-based aqueous brine comprising about 20% to about 24% of sodiumchloride, about 3% to about 7% of sodium carbonate, about 0.6% to about3% of sodium bicarbonate, and the remainder water.

References Cited in the file of this patent UNITED STATES PATENTS2,044,758 Cross et al. June 16, 1936 2,393,174 Larsen Jan. 15, 19462,474,329 Salathiel June 28, 1949 2,525,783 Farrow Oct. 17, 19502,771,419 Salathiel Nov. 20, 1956

1. A SALT-BASED DRILLING MUD FOR USE AS A CIRCULATING FLUID IN A WELL INDRILLING THROUGH ROCK FORMATIONS CONTAINING TRONA, SAID MUD CONTAININGCOLLOIDS SELECTED FROM THE GROUP CONSISTING OF CLAY AND ORGANICCOLLOIDS, AND A SALT-BASED AQUEOUS BRINE COMPRISING ABOUT 20% TO 24% OFSODIUM CHLORIDE, ABOUT 3% TO 7% OF SODIUM CARBONATE, ABOUT 0.6% TO 3% OFSODIUM BICARBONATE, AND THE REMAINDER WATER.